This invention relates generally to a replaceable filter element and, more particularly, to a filter element which is especially adapted for use in an air filter unit for agricultural tractors and over-the-road trucks. An air filter unit of this general type is disclosed in Engel et al U.S. Pat. No. 4,720,292.
The Engel et al filter unit comprises a sheet metal housing having an air filter element located therein. The filter element includes a sleeve of filter media (e.g., pleated paper) which is located between inner and outer liners made of perforated metal. One end of the filter element is closed by a circular cap formed by a potting compound which is molded around the end of the filter element in order to hold the inner and outer liners in assembled relation with the sleeve of filter media. In the Engel et al filter element, the potting compound which is used is a urethane foam having a relatively soft, rubber-like consistency.
Molded around the opposite end of the Engel et al filter element is a circular ring of urethane foam which coacts with the cap to hold the sleeve and the liners in assembled relation. The urethane ring also serves as a sealing element in that its inner diameter radially engages and is compressed by a tubular sheet metal air outlet member which is joined to the housing of the filter unit. The radial seal established between the urethane ring and the tubular housing member seals the filter element to the housing without need of compressing the filter element axially against the housing.
The filter element of the Engel et al patent has certain drawbacks. Because the urethane ring which holds the sleeve and liners together also serves as a sealing element, its axial dimension is relatively large. As a result, the ring encircles and closes off a relatively long section of the filter media and reduces the filtering capacity and service life thereof. A relatively large quantity of urethane is necessary and, in order to enable the ring to function as a sealing element, high performance urethane is required. Thus, the cost of providing a urethane sealing element is relatively high. Moreover, high performance urethane expands in a relatively dimensionally uncontrolled manner upon curing and causes the pleated paper filter media to separate in an uncontrolled fashion. This causes the filter element to have an unsightly appearance and can result in a relatively large number of elements being scrapped. In high temperature environments, difficulty may be encountered in maintaining an adequate radial seal even with high performance urethane because of the marginal compression set characteristics of such material.
The Engel et al filter element also is weak in the area of the circular end cap located at the end of the filter opposite the sealing ring. The urethane potting compound from which the end cap is made tends to deflect severely when subjected to relatively high pressure differentials and may rupture in operation.